Photos would help. I think more likely to be efflorescence than any residue from the powder.
Efflorescence happens when water travels through the grout (and sometimes even the concrete under the floor) and picks up calcium and other minerals. As the floor dries, these mineral are left on the surface. Sometimes can be removed with a stiff brushing, but more often needs to be wiped with acid. But if you use too much water, in the acid wash, you have just started the cycle over again. Wipe with damp cloth or sponge. Keep the moisture on the surface so it does not travel through the grout.
If you don't have my training manual on Stone, Tile & Grout, send an email request to
nstoker@interlinksupply.com Free copy.